A plane mirror interferometer can be used to measure the position, orientation, or movement of a precision stage in a wafer processing system. For such use, a plane mirror is typically mounted on the stage being measured, and the interferometer directs one or more measurement beams for reflections from the plane mirror. Each measurement beam generally corresponds to a separate interferometer channel, but some interferometers commonly referred to as double-pass interferometers direct each measurement beam for two reflections from the plane mirror before the interferometer combines the measurement beam with a reference beam for signal processing that produces measurements.
Multiple interferometer channels can use the same plane mirror to measure distances to separated points on a plane and thereby measure multiple degrees of freedom of a stage. In particular, the three interferometer channels measuring translations at three separate points on the same plane mirror can identify a translation of the stage along an X or beam axis, a pitch rotation of the stage about a Y axis perpendicular to the X axis, and a yaw rotation of the stage about a Z axis perpendicular to X and Y axes. Stage rotations are important to measure when the system is susceptible to Abbe errors, i.e., displacement errors due to stage rotations.
A dynamic measurement range for each channel of a plane mirror interferometer is generally limited because mirror rotations (e.g., pitch rotation) can deflect the measurement beam, causing the reflected measurement beam to “walk off” the path required for recombination with a reference beam. For some configurations, the dynamic range for measurements is roughly equal to the measurement beam radius w divided by optical path length for the measurement beam, e.g., about four times the distance L extending from the interferometer to the plane mirror in a double pass interferometer. Accordingly, the dynamic range for rotation measurements is typically limited to about w/4L radians. Increasing the beam width w can increase the dynamic measurement range. However, larger beam widths generally require larger and therefore more expensive optics, and complex systems such as wafer processing equipment may not have sufficient space for large interferometers.
A separate plane mirror interferometer can also be used to measure the position or movement of a stage along a Z axis (i.e., the focus axis of the projection lens.) However, if this interferometer is on the projection lens side of the stage, the stage must be made larger for inclusion of a plane mirror outside the projection area. This can reduce wafer throughput. Alternatively, if the interferometer is on a side opposite to the projection lens, an intermediate reference such as a stone below the stage is generally needed, which requires additional measurements of the relative position of the intermediate reference relative to the projection lens.
U.S. Pat. Nos. 6,020,964 and 6,650,419 describes interferometer systems capable of measuring an altitude of a stage. In such systems, a reflector mounted on a stage reflects a measurement beam from a horizontal incident path (along an X axis) to a vertical reflected path (along a Z axis). A reflector mounted above the stage reflects the vertically directed measurement beam back to the reflector system on the stage, where the measurement beam is redirected to a horizontal return path back to the interferometer optics. The total phase change or Doppler shift of measurement beam thus indicates a distance or movement along a path having horizontal and vertical components. A separate interferometer channel can measure the horizontal component of the path, so that the vertical component or an altitude measurement can be extracted. These altitude measurements are generally subject to the above-described dynamic range limitations at least because of the need to measure and subtract the horizontal component.
In view of the limitations of existing interferometers, systems and methods are sought that can provide a large dynamic measurement range for measurements of altitudes or vertical translations without requiring large optical elements.